1. Field of the Invention
The present invention relates generally to a thermal-type flowmeter for detecting a flow rate of a fluid by using heat-sensitive resistors. More particularly, the present invention is concerned with a signal conditioning interface circuit for a thermal-type flowmeter adapted for detecting a flow rate of intake air in an internal combustion engine, which circuit is designed for processing an output signal of a heat-sensitive flow sensor of the thermal-type flowmeter to thereby derive a detection voltage signal which indicates the flow rate and which is to undergo analogue-to-digital conversion for generating a digital signal to be supplied to an electronic control unit for the purpose of controlling fuel injection or other operation in the internal combustion engine.
2. Description of Related Art
In the thermal-type flowmeter designed for outputting a current indicating a flow rate of a fluid such as intake air in an internal combustion engine, it is known that the intake air flow signal outputted from the sensor is converted into a voltage signal for analogue-to-digital conversion by transmitting the intake air flow signal in the form of a current signal to a circuit stage preceding to an analogue-to-digital converter so that the flow rate information can be transmitted with high fidelity or reliability even if potential variation takes place in the thermal-type flowmeter and/or the electronic control unit such as the electronic fuel injection control unit.
For having better understanding of the invention, description will first be made of a conventional thermal-type flowmeter known heretofore by reference to FIG. 8 to FIG. 10, in which FIG. 8 is a circuit diagram showing a circuit configuration of a conventional thermal-type flowmeter disclosed, for example, in a Japanese Unexamined Patent Application Publication No. 216420/1990 (JP-A-2-216420), FIG. 9 is a circuit diagram showing another conventional thermal-type flowmeter, and FIG. 10 is a view for illustrating graphically input-versus-output characteristics of the thermal-type flowmeter shown in FIG. 9.
Now referring to FIG. 8, a conventional thermal-type flowmeter denoted generally by reference numeral 1 includes an operational amplifier 1b having a non-inverting input terminal (+) to which applied is a flow-rate indicating voltage signal Vafs outputted from an amplifier 1a constituting an output part of an air flow sensor (not shown) installed, for example, in an intake pipe of an internal combustion engine (not shown either). On the other hand, the output terminal of the operational amplifier 1b is connected to a base electrode of a transistor 1c while an inverting input terminal (xe2x88x92) of the operational amplifier 1b is connected an emitter electrode of the NPN-transistor 1c and one end of a reference resistor Re having the other end connected to a potential source of negative polarity. The collector of the transistor 1c is connected to a terminal of a reference voltage Vref in a fuel injection control unit 2 by way of a current detecting resistor Rc. Further, an analogue-to-digital converter (hereinafter also referred to as the A/D converter in short) incorporated in the fuel injection control unit 2 has an analogue input terminal to which the reference voltage Vref is applied by way of a current detecting resistor Rc.
In operation, when the flow-rate indicating voltage signal Vafs is inputted to the non-inverting input terminal (+) of the operational amplifier 1b from the amplifier 1a, a base current Ib flows to the base of the NPN-transistor 1c from the output terminal of the operational amplifier 1b. In that case, an emitter current Ie flows through the reference resistor Re, generating an emitter voltage Ve which is fed back to the inverting input terminal of the operational amplifier 1b. As a result of this, the emitter voltage Ve becomes equal to the voltage level of the flow-rate voltage signal Vafs.
In this conjunction, it should be mentioned that the emitter current Ie may be regarded as being equal to the collector current Ic so far as the current amplification factor of the NPN-transistor 1c is selected at a sufficiently large value. Accordingly, the input voltage Vc which is applied to the analogue input terminal of the analogue-to-digital converter and which is given by
Vc=Vrefxe2x88x92Rc. Ic
can be regarded as bearing a proportional relation to the flow-rate indicating voltage signal Vafs.
Because the flow-rate indicating voltage signal Vafs is outputted after having been converted into the collector current Ic, the intake air flow signal indicating a flow rate of the intake air can be converted into an electric signal to be transmitted to the A/D converter with high fidelity without being affected by variations of potentials which may occur in the thermal-type flowmeter 1 and/or the fuel injection control unit 2.
Next referring to FIG. 9 which shows a circuit configuration of another conventional thermal-type flowmeter 1, the flow-rate indicating voltage signal Vafs outputted from the amplifier 1a constituting a part of the sensor circuit is applied to the non-inverting input terminal (+) of the operational amplifier 1b, as in the case of the thermal-type flowmeter 1 shown in FIG. 8. The inverting input Terminal C-) of the operational amplifier 1b is connected to an emitter terminal of an NPN-transistor 1c and additionally to one end of a first reference resistor 1e which has the other end connected to the ground potential. The collector terminal of the NPN-transistor 1c is connected to a reference voltage Vcc of a power supply circuit 1d by way of a first current detecting resistor 1f. 
A voltage V2 making appearance across the first current detecting resistor 1f as a voltage drop is applied to a non-inverting input terminal of an operational amplifier 1g which has a non-inverting input terminal connected to a junction between the collector terminal of the NPN-transistor 1c and a first current detecting resistor 1f, and an output terminal connected to the base of a PNP-transistor 1h. Further, the inverting input terminal of the operational amplifier 1g is connected to an emitter of the PNP-transistor 1h. 
The emitter electrode of the PNP-transistor 1h is connected to the reference voltage Vcc of the power supply circuit 1d by way of a second reference resistor Re while the collector electrode of the PNP-transistor 1h is connected to an analogue input terminal of an A/D converter incorporated in a fuel injection control unit 2, wherein the analogue input terminal is connected to a ground potential terminal of the A/D converter by way of a second current detecting resistor Rc. Owing to the circuit arrangement described above, the flow-rate indicating voltage signal Vafs can be generated ultimately as a voltage drop Vc making appearance across the second current detecting resistor Rc. The voltage signal Vc is inputted to the A/D converter.
In operation, the current which flows through the NPN-transistor 1c and which bears a proportional relation to the output voltage of the operational amplifier 1b undergoes a current-to-voltage conversion (also referred to as the I/V conversion) through the first current detecting resistor 1f connected to the collector terminal of the NPN-transistor 1c. Thus, inputted to the non-inverting input terminal of the operational amplifier 1g is a detection voltage V2 resulting from subtraction of a voltage derived from the reference voltage Vcc through the I/V conversion, i.e.,
V2=Vccxe2x88x92I.1f.
Further, a voltage generated across the second reference resistor Re is fed back to the inverting input terminal of the operational amplifier 1g. Accordingly, the detection voltage V2 can assume a value or level which is in proportion to the level of the flow-rate indicating voltage signal Vafs.
More specifically, the output voltage of the operational amplifier 1g which is in proportion to the detection voltage V2, is applied to the base electrode of the PNP-transistor 1h. As a result of this, an emitter current Ie flows through the second reference resistor Re with the emitter voltage V3 making appearance across the second reference resistor Re, wherein the electric potential V3 is fed back to the inverting input terminal of the operational amplifier 1g. Thus, the collector current Ic which bears a proportional relation to the flow-rate indicating voltage signal Vafs is ultimately applied to the analogue input terminal of the A/D converter of the fuel injection control unit 2.
In both of the conventional thermal-type flowmeters described above, the analogue input terminal of the analogue-to-digital converter is connected to the ground potential through the second current detecting resistor Rc. Consequently, the input voltage to the A/D converter is equivalent to the voltage drop (Icxc2x7Rc) appearing across the second current detecting resistor Rc. With the arrangement in which the flow-rate indicating voltage signal Vafs is outputted after having been converted to the collector current Ic, as described above, the intake air flow signal resulting from the voltage/current conversion can be transmitted to the A/D converter with high fidelity without being affected by variations in potential which may occur in the thermal-type flowmeter 1 and/or the fuel injection control unit 2. Besides, there arises no need for providing a reference potential source in the fuel injection control unit 2.
As can be understood from the foregoing, in the conventional thermal-type flowmeters for the fuel injection control unit, the flow-rate indicating voltage signal undergoes the voltage-to-current conversion with reference to the reference voltage Vref. Accordingly, the reference voltage Vref has to be set at a high potential level. Besides, the voltage signal resulting from the current-to-voltage conversion through the second current detecting resistor Rc and inputted to the analogue input terminal of the analogue-to-digital converter has no compatibility or exchangeability with the output voltage with reference to the ground potential.
On the other hand, in the case of the thermal-type flowmeter 1 shown in FIG. 9, the reference voltage has to be set at a high potential, as can be seen from the characteristic diagram shown in FIG. 10. Furthermore, when the flow-rate indicating voltage signal is at a low potential level, the input potential for the operational amplifier becomes high. Consequently, the input circuitry for the operational amplifier has to be necessarily implemented with the NPN-transistor circuitry which is capable of inputting a high potential without being affected by the base-emitter current of the transistor.
Such being the circumstances, when the input circuitry of the other operational amplifier is implemented in the PNP-structure, two different types of operational amplifiers have to be employed, which means that not only limitation is imposed on the degree of freedom in design but also manufacturing cost will be increased. Additionally, because the reference voltage is set high, the power supply source of high potential level has to be employed, which in turn means that limitation is imposed on the selection of the power supply source to be employed, giving rise to problem.
In the light of the state of the art described above, it is an object of the present invention to provide a thermal-type flowmeter which can avoid the problems mentioned above and which can be realized inexpensively in a miniaturized structure while ensuring operation of high fidelity or accuracy even with a power supply source of a relatively low capacity.
In view of the above and other objects which will become apparent as the description proceeds, there is provided according to a general aspect of the present invention a thermal-type flowmeter for detecting a flow rate of a fluid, which includes a voltage converting means for converting a flow-rate indicating voltage signal outputted from a flow-rate detecting means into a voltage of a level falling within a predetermined range, a voltage-to-current converting means for converting the above-mentioned voltage into a current of magnitude proportional to a value of the flow-rate indicating voltage signal, a current-to-voltage converting means for converting the current into a voltage signal for analogue-to-digital conversion, and a voltage adjusting means for increasing or decreasing output of the voltage converting means in dependence on the value of the flow-rate indicating voltage signal.
By virtue of the arrangement of the thermal-type flowmeter described above, stabilized voltage-to-current conversion can be achieved independent of the value or level of the flow-rate indicating voltage signal.
In a preferred mode for carrying out the invention, the voltage converting means may be so designed as to include a gain adjusting means for changing amplification factor for the flow-rate indicating voltage signal inputted to the voltage converting means.
With to the arrangement of the thermal-type flowmeter described above, stabilized voltage-to-current conversion can be achieved nevertheless of variation of independent of the value or level of the flow-rate indicating voltage signal.
In another preferred mode for carrying out the invention, the voltage adjusting means may be constituted by a series circuit of a resistor or resistors, a diode or diodes, a Zener diode or combinations thereof.
With the arrangement of the thermal-type flowmeter, the voltage adjusting means can be realized inexpensively, to an advantageous effect.
In yet another preferred mode for carrying out the invention, the thermal-type flowmeter may further include a current adjusting means provided in association with a current output part of the voltage-to-current converting means for adjusting a current value of the current output part.
Owing to the circuit arrangement described above, dispersion of the output current brought about by dispersion of circuit constants can be canceled out satisfactorily, whereby stabilized current-to-voltage conversion can be ensured.
In still another preferred mode for carrying out the invention, the current output part may be implemented in the form of a transistor circuitry including two transistors interconnected in the form of a Darlington circuitry.
With the arrangement of the thermal-type flowmeter described above, fluctuation of the base current of the transistor can be suppressed.
In a further preferred mode for carrying out the invention, the current adjusting means may include a constant current circuit for adding a constant source current to the output current at a low-voltage side of the current output part.
Owing to the circuit arrangement of the thermal-type flowmeter described above, stabilized output signal can be obtained without being affected by the gain of the transistor circuit.
In a yet further preferred mode for carrying out the invention, the current adjusting means may include a constant current circuit for adding a constant sink current to the output current at a high-voltage side of the current output part.
With the circuit arrangement of the thermal-type flowmeter described above, stabilized output signal can be obtained without being affected by the gain of the transistor circuit.
In a still further preferred mode for carrying out the invention, the voltage converting means may be so designed as to convert the flow-rate indicating voltage signal into a voltage of the level falling within the predetermined range after effecting current amplification of the flow-rate indicating voltage signal.
With the arrangement of the thermal-type flowmeter described above, the current consumption of the voltage converting means can be diminished, to a further advantage.
The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings.